1. Field of the Invention
The present invention relates to switching power supply units outputting a DC current and a DC voltage.
2. Description of the Related Art
FIG. 5 shows an example of a prior art main circuit structure of a switching power supply unit. The switching power supply unit shown in FIG. 5 is a forward converter in which a transformer 1 is disposed. A primary coil 2 of the transformer 1 is electrically connected to a primary-side circuit having a main switching element 3 (an N-channel MOS-FET in the example shown in FIG. 5) and a capacitor 4, and a secondary coil 5 is electrically connected to an output-side circuit (a secondary-side circuit) 10 having a rectifying diode 6, a commutating diode 7, an inductor 8, and a capacitor 9.
In addition, the gate of the main switching element 3 is electrically connected to a switching control circuit 11 controlling the on-off switching operation of the main switching element 3. Furthermore, an output voltage detection unit 11a and an output current detection unit 12, which will be described below, are also provided as shown.
There is provided a structure in which a DC current and a DC voltage are inputted to the primary-side circuit via input terminals 13a and 13b. As already known, the inputted DC current and DC voltage are converted into an AC current and an AC voltage by the on-off switching operation of the main switching element 3, and then, the AC current and AC voltage are outputted to the output-side circuit 10 from the secondary coil 5 of the transformer 1.
In the output-side circuit 10, the AC current and AC voltage are rectified and smoothed by using a choke-input rectifying method, and the rectified and smoothed DC current and DC voltage are outputted as an output current Iout and an output voltage Vout of the switching power supply unit to a load 15 via output terminals 14a and 14b.
The output voltage detection unit 11a shown in FIG. 5 directly detects the output voltage Vout of the switching power supply unit outputted to the load 15 from the output-side circuit 10, and then outputs a voltage corresponding to the output voltage Vout to the switching control circuit 11. The switching control circuit 11 controls the on-off switching operation of the main switching element 3 so that the output voltage Vout is outputted with a specified voltage value in a stabilized manner based on the applied voltage.
As shown in FIG. 5, the output current detection circuit 12 comprises a current transformer 16, a rectifying diode 17, a resistor 18, and a capacitor 19. The current transformer 16 is disposed on a current-flowing path through which the same current as a drain current of the main switching element 3 flows. The drain current of the main switching element 3 is converted into a voltage and is amplified by the current transformer 16 and the resistor 18. The voltage is rectified by the rectifying diode 17 and then is charged in the capacitor 19.
The value of the drain current of the main switching element 3 periodically changes. A current value at peak (hereinafter referred to as a peak current value) during a period as a cycle in the drain current of the main switching element 3 is a current value corresponding to the output current Iout of the switching power supply unit. Since the charging voltage value of the capacitor 19 is approximately equivalent to a voltage value corresponding to the peak current value of the drain current of the main switching element 3, the charging voltage of the capacitor 19 is also a voltage corresponding to the output current Iout of the switching power supply unit. The output current detection circuit 12 outputs the charging voltage of the capacitor 19 as a voltage corresponding to the output current Iout.
In the example shown in FIG. 5, the output voltage of the output current detection circuit 12 is applied to the switching control circuit 11. When the switching control circuit 11 detects that the output current Iout is in an overcurrent state due to an abnormal condition of the load 15 or the like based on the output voltage of the output current detection circuit 12, it controls the switching of the main switching element 3 in such a manner that the output current Iout decreases so as to eliminate the overcurrent state, by which the switching power supply unit is protected.
In the example shown in FIG. 5, the output current detection circuit 12, as described above, has a circuit structure of detecting and outputting the voltage corresponding to the output current Iout by using the current transformer 16. However, since the current transformer 16 is an expensive and large component, cost reduction and miniaturization of the switching power supply unit cannot be achieved.
Thus, an output current detection circuit in which a resistor is disposed as an alternative to the current transformer 16 has been provided. However, in this case, there is a large amount of conduction loss of the resistor, which leads to a problem in that the circuit efficiency of the switching power supply unit deteriorates.
Regarding this case, the reason for causing such a problem will be described as follows. In the aforementioned output current detection circuit having the resistor as an alternative to the current transformer 16, a diode is used for rectification. When the voltage applied to the resistor is approximately equal to the forward voltage drop of the diode, the output voltage outputted from the output current detection circuit changes according to changes in temperature due to a negative influence of dropping of the forward voltage of the diode, regardless of changes in the output current Iout, which is a problem of temperature drift.
In order to prevent such a problem of temperature drift, there is provided a structure in which a resistance value of the resistor is set to be large so that the voltage applied to the resistor is significantly larger (for example, 5 to 6 V) than the forward voltage of the drop of the diode. As a result, as described above, the conduction loss in the resistor significantly increases, and this leads to the problem that the circuit efficiency of the switching power supply unit deteriorates.